closeOnBackpressure should block all further sending

[ghost]

When we reach maxBackpressure we start closing the connection (which doesn't even work on Windows!). But there is no guarantee that backpressure won't drain a bit and we then send another message still. This can cause a gap in the stream. This needs to be guaranteed that when we reach maxBackpressure and we are closing, never send anything more. To guarantee stream integrity.